Brazil and Paraguay have a long history of tense relations permeated by conflict, suspicion, and war. This situation only eased with the rise of military dictatorships in both countries during the second half of the twentieth century and particularly as they embarked together in the construction of the biggest infrastructure project in the hemisphere: the Itaipu Dam. This chapter explores how two South American dictatorships launched national-developmentalism with the construction of a mega infrastructure in the 1970s. According to the national-developmentalism’s playbook, nature was a resource to fuel development and in the case of the Itaipu Dam on the Paraná River, it also provided a common ground that allowed for the settlement of a diplomatic conflict. We also explore the construction of the Itaipu Dam to understand how right-wing dictatorships grasped and dealt with the rise of environmentalism in the 1970s. This chapter furthermore deals with the emergence of both environmental legislation and organized struggles in these two countries.